All the separations have the same LPI as defined in the Odystar or Nexus workflow, except for Yellow. Why is this?
This can occur when using Paragon screens. AM screens such as Paragon screens are arranged in 30 degree increments to avoid interference effects between the separations.
The problem with this is that, it is not possible to fit all four screens in 90 degrees if they are all 30 degrees apart. To counter this, Paragon screens tend to put Yellow at an angle which falls between two other angles. In the above example, the angle of Yellow is different by 15 degrees to the Cyan and Black angles. Yellow was chosen because it is the lightest of the process colors, and any moire which occurs on the Yellow separation will be less visible than in the other colors.
However, moire could still occur unless other methods are used to prevent it. Hence Paragon screens define the frequency of the Yellow separation to be slightly different from the other separations. This allows the Yellow dots to be placed in such a way that they will not interfere with the dots of the other separations. This frequency shift is calculated as part of the screen itself and cannot be altered by the user.
In the above example, if the screen frequency was set to be a (nominal) 150 lpi, the Cyan, Magenta and Black separations would be screened with a ruling of ~150 lpi, while the frequency of the Yellow separation would be adjusted to ~168 lpi.